The present disclosure relates to a combined MRI and radiation therapy system. In particular, it relates to such a system which is compact, inexpensive and employs a background magnetic field for MRI imaging with high magnetic flux.
Recently, attempts have been made to combine imaging systems with therapy systems, particularly in the field of radiation therapy, as such combined systems allow localization of tumors as, or immediately before, the treatment beam is applied. This ensures that the treatment beam is correctly targeted, in turn meaning that treatment may be more effective and that unintentional irradiation of healthy tissue is minimized.
Certain radiation therapy systems utilize highly penetrating gamma-like radiation to kill cancerous tissue. Gamma radiation is generally regarded as electromagnetic radiation having a wavelength of between 10−10 m and 2×10−13 m, or quantum energy in the range 104 eV to 5×106 eV. High energy x-rays also fall within this range, and the present description should be understood in the sense that “gamma radiation” includes all electromagnetic radiation of sufficient energy to be useful in radiation therapy applications.
Gamma radiation is not perturbed by magnetic fields and can only be screened by the use of significant amounts of dense material such as lead or concrete. Radiation of this type is normally generated by either small linear accelerators or by gamma-emitting radioactive sources such as cobalt-60. Since linear accelerators are affected by background magnetic fields, the second of these options is preferred in the present preferred embodiment, as the magnetic field required by the MRI system will not interfere with the generation of gamma radiation using a radioactive source.
Previously, separate MRI and radiation therapy systems were used but this was found to be far from ideal. Problems with organ motion and image registration led to poor utilization of the available radiation dose and accidental necrosis of viable tissue.
More recently, some superconducting magnet configurations have been developed for combined MRI and radiation therapy applications, using split magnets with a rotating gamma source in the gap between the two parts of the magnet. The resulting complex, cumbersome designs have relatively low field and poor homogeneity due to the necessarily large axial distance between the center-most coils of the magnet. The need to accommodate a rotating Gamma source in the gap means that the problem of supporting the two cryostat halves with respect to each other presents many difficulties. Mechanical difficulties associated with restraining the forces generated between the two halves of the magnet lead to further cumbersome arrangements.